MAGNESIAN-LIMESTONE OP THE COUNTY OF DT7EHAM. 63 



thick ill-defined concentric layers, which are either aggregated 

 about an earthy nucleus or are hollow in the centre (in the 

 latter case the earthy nucleus has probably been afterwards 

 carried off). These layers are of various colours. Towards the 

 centre the fracture is generally dull and earthy, but towards 

 the circumference the layers are made up of many thin crystal- 

 line plates, and the lustre is shining. In other instances balls 

 of considerable size have the internal structure here described, 

 while the outer zone is made up of diverging crystalline fibres, 

 with the usual acumination. The transverse fracture in such 

 cases is very beautiful." 



"The spheroidal concretions are in some places subordinate 

 to the pulverulent matter ; in others they abound so much that 

 they nearly fill the irregular cells ; and the ochreous powder 

 appears as an upfiUing matter in the intervening spaces. In 

 some rare instances the earthy matter becomes hard and coherent, 

 and when broken with a hammer exposes a surface which passes 

 through the centre of the imbedded balls." 



The rock structure alluded to in these extracts, and to which 

 more, particularly it is wished to direct attention, is made up 

 almost entirely of spheroidal balls of various sizes imbedded in 

 a loose powdery yellowish marl, which is in some instances 

 finely laminated and arranged in layers between the spheroidal 

 compact balls. As stated before, these spheres occur occasion- 

 ally perfectly isolated in the powdery marl ; in some other 

 instances they are joined together, and more or less attached 

 to others in some part of their circumference. Most of these 

 when split asunder are homogeneous in structure and finely 

 crystalline. Others have a hollow centre, the nucleus having 

 been removed; and in some the parts near the circumference 

 shew a radiating crystalline structure. They are all more or 

 less globular, but this shape is modified a little in the balls 

 which occur in the lowest part of the bed, where they are flatter 

 and more stalagmitic in appearance and shape. In the upper 

 part of the bed, the concretions are attached to the base of the 

 superior stratum, and are of a semispheroidal or mammillated 

 form, and have the appearance of flattened stalactites. In some 



